Method of continuous manufacture of membrane filters



rates atent @fi 3,026,57l Patented Mar. 27, 1%62 3,026,571 METHOD OFCONTHNUOUS MANUFACTURE OF MEMBRANE FHLTERS Karl Heinrich Maier,Gottingen, Germany, assignor, by mesne assignments, toMembranfiltergesellschaft G.m.b.H., Gottingen, Germany, a corporation ofGer- Filed Oct. 23, 1958, Ser. No. 769,978 Claims priority, applicationGermany July 4, 1956 6 Claims. (Cl. 18-57) The present invention relatesto a method of and apparatus for the continuous manufacture of membranefilters.

Membrane filters are microporous filters of a high degree of finenesswhich are suitable for filtering of microorganisms or similar finegrained material in a filtering process.

This is a continuation-in-part application of the copending applicationSer. No. 634,588, filed January 16, 1957, now abandoned.

It is known that such membranes may be produced from solutions ofcellulose esters, particularly of cellulose nitrate and celluloseacetate in volatile organic solvents. In order to achieve this end aviscous solution of cellulose ester is poured or drawn on plain glassplates in form of a layer, so that the glass plates are covered with aviscons film, the thickness of which may be predetermined by providing aweir. In this process the solution is fed into a so-called draw slide,one wall of which is formed as a thin weir and upon drawing of the slidethe liquid film is equally distributed in the form of a strip over theglass plate. After the liquid film is applied, the solvent mixture isevaporated, whereby, a solidification of the liquid film to a dry foilhaving artificial material-like characteristics and simultaneously offine porous structure is brought about.

It is already known in the manufacture of membranes not to use puresolvents, but mixtures of esters, ketones or ether on the one hand andlow alcohols, as ethyl alcohol, butyl alcohol and amyl alcohol on theother hand. The composition of the mixtures, sometimes also withaddition of glycerine or small quantities of water, determinessubstantially the degree of fineness of the formed membrane film. Inthis connection there is a distinction between solvents of a first orderwhich have the characteristic of a solvent (ester, etc.) for celluloseester and solvents of a second order (alcohols) which have thecharacteristic of a precipitant for cellulose ester. During theevaporation a shift of the relation of the two solvents in favor of theprecipitant takes place, which leads to the gelation and later followingsolidification of the membrane. Thus this used clean evaporation processhas the drawback that a comparatively long time is required in order toevaporate all materials capable of evaporation out of the membrane filmand for this reason a continuous manufacturing process is not possible.Furthermore, the machinery for performing this known process requires anunusually large space, since the glass plates carrying the film areexposed for 8 to 12 hours, as has been established by experience. Ashortening of the process by preliminary solution of the membranes isnot feasible, since the remainder of the solution ingredients would leadto later shrinking and fold formations, which exclude the later use ofthe final product as a filter.

It is, therefore, one object of the present inventionto provide a methodfor the continuous manufacture of membrane filters which eliminates thedrawbacks inherent in the long development time of the membrane in theold processes.

It is still another object of the present invention to provide a methodof the continuous manufacture of membrane filters in which the timerequired for the forma-.

tion of the film is decidedly shortened by removal of the solutioningredients in such manner that this removal step is divided into twophases. light volatile solvent (solvents of the first order) evaporatesat a temperature within a range of 20 to 40 C. and at a relativehumidity of 50% to 70% and in the second phase the heavier volatilesolvent ingredients (solvents of the second order) are removed by meansof diffusion in a liquid bath of a temperature within a range of 20 to40 C. from the membrane. It has been found that in this manner the timefor removal of all solvents and softener additions may be reduced from aplurality of hours to a plurality of minutes.

Liquids are suitable as liquid baths which have the characteristic of aprecipitant for the membrane mass and which are simultaneously mixablewithout limitation with the solvents of the second order as ethylalcohol, butyl alcohol and glycerine. Water or water solutions ofalcohols or esters may be used advantageously for this purpose.

The following examples are given therefor:

(l) Distilled water at a temperature of 35 C.; and (2) A 2% watersolution of glycerine at a temperature of 35 C.

The evaporation of the light volatile solvents may be appreciablyaccelerated by airing combined with the selection of the most suitableclimate. Also the diffusion of the heavier volatile solution ingredientsfrom the membrane in a liquid bath may be controlled by using the wellknown reverse current principle in such manner that the diffusiondifference is always at its optimum. Due to the combination of these twomeasures is it possible to reduce the time required for the membraneformation to a degree that a continuous manufacturing process is made.

The following two examples are given as solutions for the layer, whichare suitable in accordance with the present invention:

(1) In order to form the solution for the layer 3 kg. of marketedcellulose nitrate of a quality suitable for highly viscous solutionswith a content of 33% ethyl alcohol is dissolved by stirring in thefollowing mixture:

22 1. methyl acetate 10 l. ethyl alcohol 5 l. butanol There is made anaddition to this solution of- 0.7 l. glycerine 0.8 l. distilled waterAfter standing for a period of 24 hours for removal of the air bubbles,the viscous solution is ready for use.

(2) 2.4 kg. cellulose nitrate, of the same quality, as disclosed inExample 1, with a content of alcohol of 33% and 0.4 kg. of marketedcellulose acetate which is airdry are dissolved in the same mixturestated above in connection with Example 1 by means of stirring. Thereis' added to this solution:

1 l. glycerine 1 1. water.

With these and other objects in view which will become apparent in thefollowing detailed description, the present invention will be clearlyunderstood in connection with the accompanying drawing, in which theonly figure is a diagrammatic showing of the apparatus for performingthe method in accordance with the present invention.

The apparatus, which is shown in the drawing by example only, comprisesa storage container 4 from which the layer solution is fed through aconduit 5 controlled In the first phase the by a control valve 6 intothe layer weir 7. The solution is then fed from the latter through aslot onto the endless ribbon 1 which runs over the return rollers 2 anda supporting roller 3. The solution 8 which has been applied to theribbon 1 passes at first the preliminary chamber 10 and then into theairing chamber 11 in which the pregelatiou step takes place. An airstream passes through the airing chamber 11, which air stream enters atthe opening 12 and leaves at the opening 13 with simultaneous removal ofthe light volatile solvent. The ribbon 1, which carries now the membranein its pre-gelation state is then fed into a diffusion bath container 14over the return rollers which bath is supplied at the opening 20 of thecontainer 14 and which leaves the container 14 at the opening 21 andthus receives continually fresh bath liquid. The finally developed moistmembrane 9 is then fed to a removal drum 15 and over turning rollers andthen over heating drums 16 and finally passed through a drying channel18. The removal drum 15 serves the purpose to remove the moist membranefilm from the endless band.

The drying channel 18 receives hot air by provision of an airing system17 which hot air strikes over the membrane on both sides thereof. Afterpassing the drying channel 18 the finished membrane film is wound on adrum 19.

In addition to the mechanical means for removal of the moist membranefilm from the endless ribbon I, as the removal drum 15, chemical meansmay be provided also. Such chemical means may be softeners, removers orseparating oils, which may be used selectively for treatment of the bathliquid, the solution of the layer or the surface of the endless band.

For the chemical additions to the bath liquid, the above statedsolutions for the diffusion bath are sent through the bath by using thereverse stream principle. Advantageously the content of the container 14receiving about 3.5 m. of liquid is changed within a period of 4 hours.The flow through the container is properly controlled and adjusted byconventional flow meters. Simultaneously it is possible by use of waterwith additions of alcohols or esters to control and to adjust the amountof the additions by means of flow meters or known dosage devices.

As an example it may be stated that if a flow of 15 1. of water takesplace per minute, an addition of 0.3 l. of glycerine is made per minute.It is also possible to provide an addition of 1 l. of methyl alcohol perminute in case of a flow of 15 l. of water per minute.

The layer solution may be treated by example in the following manner inorder to remove the layer from the endless band:

In the above mentioned layer solutions of pure cellulose nitrate ormixtures of cellulose nitrate and cellulose acetate, the glycerineaddition is increased from 0.7 l. to 1.5 l. in the first example andfrom 1 kg. to 2 kg. in the second example.

Finally, in order to bring about the removal of the layer, the surfaceof the endless band may be treated by chemical means.

In order to increase the capacity of removal, a thin layer of separatingoil (silicone oil) is continuously applied. For this purpose, a spongeor a felt drum (not shown), soaked with separating oil, may be providedadjacent the layer weir 7.

It has been set forth above that the composition of the mixturesdetermines the degree or the porosity of the membrane filter incombination with the temperature and relative humidity ranges appliedduring the forming process.

The factors which are to be considered in the composition of thesolution are of colloidal chemical nature, while the temperature andrelative humidity ranges are climatically effective factors. Inconsidering the colloidal chemical factors, not only the combination ofthe sufficient quantities as economical raw material.

different solvents, but also the characteristics of the used solidmaterials, which are in the present case the cellulose esters, must beconsidered.

Practically, nearly exclusively cellulose nitrates and celluloseacetates are used in the manufacture of membranes, since these celluloseester types are available in Preferably ester-soluble nitro wool, whichcorresponds with the nitro wools E1440 to E730 marketed in Germany byWolff & Co., Walsrode, Germany or equivalent products of German orforeign manufacture are used. The products E1440 to E730 correspondclosely with the types RS250 to 400 sec., to RS 5-6 sec. of HerculesPowder. These products, which differ from each other by their differentdegrees of polymerization and different viscosities, may be used in themanufacture of membrane filters alone or in mixtures with otherproducts.

As an example for the mixture of wools of different degrees ofpolymerization, the manufacture of very coarse porous membrane filtersfor air filtration is given. In this case, a nitro wool of the typeE1250 is admixed with a nitro wool E730 with a proportion of 2:1, inorder to achieve a higher content of solid material and, thereby, tobring about a better mechanical rigidity of the very loosely formedmembrane filter.

The used layer solution has by example the following composition:

A membrane filter is obtained with this solution the air penetration ofwhich is 6 liters of air/cm. /min./ 500 mm. of water-column. The medianpore width is S-IO t.

As technically suitable acetates, commercially available celluloseacetates, as Cellit of the type L1200 to L900, marketed byFarbenfabriken Bayer, Leverkusen, having a content of 63% of aceticacid, may be used. These cellulose esters may be used alone or inmixtures, thus as cellulose nitrate or cellulose acetate alone, as wellas such nitrate-acetate mixtures, in order to produce relatively porousand well working membrane filters. The examples set forth above apply inthis case.

Example I for cellulose nitrate alone:

3 kg. cellulose nitrate having a content of 33% ethyl alcohol 22 1.methyl acetate 10 l. ethyl alcohol 5 l. butyl alcohol 0.7 l. glycerin0.8 1. distilled water Example II for the mixture of cellulose nitratewith cellulose acetate:

2.4 kg. cellulose nitrate having a content of ethyl alcohol of 33% 0.4kg. cellulose acetate, air dry 22 1. methyl acetate 10 l. ethyl alcohol5 l. butyl alcohol 1 kg. glycerin l 1. Water Example III for celluloseacetate alone:

2.8 kg. cellulose acetate 40 1. methyl acetate 20 l. isobutyl alcohol0.6 l. glycerin If the layer solutions given in the Examples I to IIIare used, membrane filters of the type for bacteria filter having amedian pore width of about 0.5 micron are achieved.

The concentration of the solid material content (cellu- In order todemonstrate the influence of the water con-- tent on the solvent, thefollowing examples are given:

lose ester) is determined by the gelation behavior and the (a) lkmbraneter AF for an filtratlon' Percent ellulose nitrate 6 viscosity of thesolution. These two factors limit the Methyl acetate 53 technicalpossibilities for the prototype for the production E11 1 al C Ohol' ofthe membrane filter E1250 to a concentration range i al C oh 12 of about3% to about 15%. If a percentage below said G1 scerin 1 3 range is used,no continuous gelatfine can 1llae achieved. Wgter Solutions having aconcentration 0 more t an 15% do 10 not lend themselves for use as alayer due to their extreme Medlal pore dlameter about 540 mlcmns'toughness and do not provide an even surface. Tech- Membrane l r C arsnically particularly favorable characteristics are found C llulosenitrate 5.5 in layer solutions having a percentage of 5% to 6% cellu-Methyl acetate 58.1 lose nitrate. 15 Alcohol 21.8 Reference is madeabove to the difierent types of sol- Butyl al ohol 11.6 vents, namelysolvents of the first order and of the second Glycerin 24 order. Theformer are known in the manufacture of Water 0.6 lacquer as realsolvents for nitro wools, while the latter Median pore diameter about0.4 micron. are known as so-called swelling means for nitro cellulose. It. ea, As solvents of the first order have been named by exam- (0) $525:; gg medlan 5 94 ple esters, ketones and ether. Suitable solvents fornitro Mthyl acetate 1 wools in particular are methyl acetate, ethylacetate, amyl Ethyl alcchn] acetate, acetone, methyl-ethyl ketone, andothers. These glityl alcohol solvents are exchangeable among each otherwithin far 25 Glycerin I 4 limits. It is merely necessary to provide thecondition Water :IIIIIIItTi: O1'6 ghzzetlgygflayre good solvents forcellulose ester and evap- Median pore diameter about 025 micron.

The same solvents cited for cellulose nitrate are likerane filter fine:I wise applicable for cellulose acetate, and for the latter The mingredients as ill however 110 Water may be added methylen chloride,particularly in mixtures addition- Median P0136 diameter about 0- m withbutyl alcohol or ethyl alcohol as good solvents. If cellulcse acfiliaiesam used, the Water As solvents of the second order may be particularlyaddiliml has the following effect! cited ethyl alcohol, butyl alcoholand amyl alcohol. If cellulose acetates are used, in addition alsoisobutyl alco- Water addition Median Ore hol has been found suitable.percent diametefin In the latter case the following example is given:

a. -2 2.8 kg. Cellit L1000 3' 3212 36 l. methylene chloride g g z g figiC impared with the decisive elfect caused by the change 0.6 L glycerinof the composition of the solvent, the 1nfiuence of the climatic factorsis gradual only. As a general rule, it may be stated that in order toachieve a fine structure a The relation of quantities of solvents of thefirst order l h idi i hi h range f 5 0% is to be used, and solvents ofthe Second Order 18 generally 3 while in order to achieve coarserstructures a higher huchanging this relation, a gradual change of thefineness of idi of 6() 7()% i to be d the membrane filter y be broughtabout The follOW- The following example indicates the effect of a varieding example is given on that point. h idi In using the basic compositionof the layer solution according to Example I, by changing the solventcompo- Hams the following effect may be achieved: Relative humidity at20o. iiiii iii rgi il iil nsec. ater in p. Relation of solvent of thefirst order to solvent filtration median 50. -80 about 0.4 of the secondorder time in pore diarn- 60 40-50 about 0.5 sec. eter in [.L 70 25-30about 0.6

3:2". 25 0.6 an 60 0.4 60 The Influence of the an temperature upon thedegree of porosity is within limits less recognizable than the influence of the relative humidity. It is for instance possible The givenfiltration time relates to a usable filter area to Produce CoarseStructures withinu the limit of the of 12.5 cm. a quantity of Water of0.51, with a pressure vegnllonahmom temperatures of 20 to Upondifference of 70 Hg raising this range to the range of 30 C. to 40 C.unde- If now the relation of the Solvents 0f the fi t and sir-ableirregularities in the structure are increasingly exond orders is changedto the extent that only solvents of P P as Well a lack of PE Y ofPenetration is the first order are contained in the mixture, glass-clearIn the resugtmg i material filters are achieved, which do not permit anypenetration, It IS undoubteflly qulte apparent from the dascl'lptlon andWhere practically no porosity can be Shown set forth above in whatmanner and to what extent the If the addition of solvents of the secondorder operates the formation of a certain porosity and retains looselythe formed gelatinous membrane, an addition of water as precipitant forcellulose nitrate has a particularly drastic effect.

manufacture of membrane filters on the basis of cellulose esters isdependent upon colloid chemical factors and in what manner theconditions in the manufacture of membrane filters are distinguished overthose processes which are aimed at the manufacture of photofilms orcelluloid l foils. In the manufacture of membrane filters the obtentionof certain physical characteristics, particularly the formation of acertain pore structure is of greatest essence.

While I have disclosed several embodiments of the present invention, itis to be understood that these embodiments are given by example only andnot in a limiting sense, the scope of the present invention beingdetermined by the objects and the claims.

I claim:

1. A process of making a porous diaphragm filter comprising the steps ofcontinuously coating a moving endless belt with a solution of 3% to of acellulose ester selected from the group consisting of cellulose nitrateand cellulose acetate dissolved in an organic solvent containing lightvolatile components selected from the group consisting of ethyl acetate,amyl acetate, acetone, methylethyl ketone, ether, ketone, ester, andheavier volatile components selected from the group consisting of ethylalcohol, butyl alcohol, amyl alcohol and isobutyl alcohol to form acoating layer, feeding said belt coated with said solution into achamber maintained at a relative humidity of from 50% to 70% and atemperature of from C. to 40 C; in order to obtain a tgelatinization anda vaporization of said light volatile components of said solution;feeding said belt through a bath selected from the group consisting ofpure water, an aqueous solution of alcohols, an aqueous solution ofesters and a glycerin solution at a temperature of from 20 C. to 40 C.in order to solidify said coating layer on said diaphragm filter and toremove said heavier volatile components of said solution; and separatingthe formed porous diaphragm filter from said belt after emerging fromsaid bath.

2. The process, as set forth in claim 1, which includes the step ofadding glycerin to said cellulose ester and said organic solvents.

3. The process, as set forth in claim 1, which includes the step ofadding a small quantity of Water to said solution of cellulose estersand said organic solvents.

4. The process, as set forth in claim 1, wherein said gelatinization andvaporization step of said readily volatile solvent is carried out in anatmosphere flowing counter to the movement of said belt.

5. The process, as set forth in claim 1, wherein the liquid in saidchamber moves in counter-current relationship to the movement of saidbelt, in order to maintain a constant diffusion gradient.

6. A process of making a porous diaphragm filter comprising the steps ofcontinuously coating a moving endless belt with a solution of 3% to 15%of cellulose acetate dissolved in methylene chloride and heavycomponents selected from the group consisting of ethyl alcohol, butylalcohol, amyl alcohol and isobutyl alcohol; feeding said coated beltinto a chamber maintained at a relative humidity of from to and atemperature of from 20 C. to 40 C. in order to obtain a gelatinizationand a vaporization of said readily volatile components of said solutionto form a coating layer, feeding said belt through a bath selected fromthe group consisting of pure water, an aqueous solution of alcohols, anaqueous solution of esters, and a glycerin solution at a temperature offrom 20 C. to 40 C. in order to solidify said coating layer on saiddiaphragm filter and to remove said heavy volatile components of saidsolution; and separating the formed porous diaphragm filter from saidbelt after emerging from said bath.

References Cited in the tile of this patent UNITED STATES PATENTS1,351,652 Jarvis Aug. 31, 1920 1,421,341 Zsigmondy June 27, 19221,576,523 Looney Mar. 16, 1926 1,845,377 Wells Feb. 16, 1932 1,878,559Wells et al. Sept. 20, 1932 2,045,370 Roberts et al. June 23, 19362,203,596 Konig June 4, 1940 2,218,146 Fothergill Oct. 15, 19402,245,429 Carver et a1 June 10, 1941 2,253,157 Weingand et al. Aug. 19,1941 2,308,949 Alderfer Jan. 19, 1943 2,714,745 Kenyon Aug. 9, 1955

1. A PROCESS OF MAKING A POROUS DIAPHRAGM FILTER COMPRISING THE STEPS OFCONTINUOUSLY COATING A MOVING ENDLESS BELT WITH A SOLUTION OF 3% TO 15%OF A CELLULOSE ESTER SELECTED FROM THE GROUP CONSISTING OF CELLULOSENITRATE AND CELLULOSE ACETATE DISSOLVED IN AN ORGANIC SOLVENT CONTAININGLIGHT VOLATILE COMPONENTS SELECTED FROM THE GROUP CONSISTING OF ETHYLACETATE, AMYL ACETATE, ACETONE, METHYLENTHYL KETONE, ETHER, KETONE,ESTER, AND HEAVIER VOLATILE COMPONENTS SELECTED FROM THE GROUPCONSISTING OF ETHYL ALCOHOL BUTYL ALCOHOL, AMYL ALCOHOL AND ISOBUTYLALCOHOL TO FORM A COATING LAYER, FEEDING SAID BELT COATED WITH SAIDSOLUTION INTO A CHAMBER MAINTAINED AT A RELATIVE HUMIDITY OF